how to set the scene for a low conflict divorce

See a lawyer in your jurisdiction to establish how a divorce and child custody is likely to shake out for you.
Fully consider that advice...you are not obligated to proceed with it.

If you are going to proceed with it, then you need to tailor an exit strategy within those legal parameters. Where you're going to live etc etc etc etc etc. Get it knocked into do-able shape.
Fully consider that strategy...you are not obligated to proceed with it.

Shore up your support network, those who can support you through the tough times that lie ahead. Consider a counsellor to be part of your support network.

Research everything you can find on helping kids transition through such an event. Maybe counselling for them given how nuts their mother is. And presumably you are going to try and get full custody of the kids.

In short, get yourself as fully prepared as you can.

Given what you have said about your hair-trigger temper missus, your chances of an amicable process is extremely poor. This new information about her overspending does not bode well for an amicable division of assets either.

The kids custody issue just about guarantees you that the process will have to be heard by a judge, the chances of negotiating this by yourselves is practically non-existent.

I think it would be wise to start mentally preparing for a shit fight.

But, one step at a time.
Get your legal advice.
Then consider the next bit.